Packing materials for liquid chromatography (LC) are generally classified into two types: organic materials, e.g., polydivinylbenzene, and inorganic materials typified by silica. Many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials not only lack the mechanical strength of typical chromatographic silicas, but also shrink and swell when the composition of the mobile phase is changed.
Due in large part to these limitations, silica is the material most widely used in High Performance Liquid Chromatography (HPLC). The most common applications employ silica that has been surface-derivatized with an organic functional group such as octadecyl (C18), octyl (C8), phenyl, amino, cyano, etc. As stationary phases for HPLC, these packing materials result in columns that have high efficiency and do not show evidence of shrinking or swelling.
To overcome the problems of residual silanol group activity (i.e., increased retention, excessive peak tailing and irreversible adsorption of some analytes) and hydrolytic instability of silica-based stationary phases, many methods have been tried including use of ultrapure silica, carbonized silica, coating of the silica surface with polymeric materials, endcapping free silanol groups with a short-chain reagent such as trimethylsilane, and the addition of suppressors such as amines to the eluant. These approaches have not proven to be completely satisfactory in practice.
One approach disclosed in U.S. Pat. No. 4,017,528 relates to a process for preparing a “hybrid” silica, wherein an alkyl functionality is coupled into both the skeleton structure and the surface of the silica. However, numerous problems remained prevalent with these materials, including packing problems associated with inhomogeneous morphology, irregular shapes and high concentrations of micropores that inhibit solute mass transfer, resulting in poor peak shape and band broadening.
Although additional approaches have been used to circumvent these problems, there still remains a need for alternative materials that solve these problems as well as provide additional advantages, for example, mechanical strength, increased column efficiency, and chromatographic selectivity.